1. Field of the Invention
This invention relates to audio data communications.
2. Description of the Prior Art
Various standards have been established for communication of digital audio data by audio equipment. Two-channel, or stereo, devices generally conform to the AES/EBU standard defined in AES3-1985 (J. Audio Eng. Soc., Vol 33, No 12, 1985, 975-984). This standard provides for the serial digital transmission of two channels of periodically sampled audio signals, two samples of audio data, one from each channel, being transmitted in each sample period. For communication between multi-channel devices, for example between a digital mixer and digital multi-track tape recorders, the Multi-Channel Audio Digital Interface standard (MADI) has been developed. This standard is defined in AES10-1991 (J. Audio Eng. Soc., Vol 39, No 5, 1991, 368-377). The MADI format is based on the AES/EBU two-channel format but provides for the serial digital transmission of 56 channels of periodically sampled audio signals. Briefly, digital audio data is formatted in frames, each of which consists of 56 data blocks or "channels". Each channel consists of a set of audio sample data relating to one signal accompanied by related data bits. The complete frame is transmitted in one period of the source sampling frequency, so samples of up to 56 audio signals can be transmitted in each sample period.
While MADI provides for communication of 56 channels of audio data, only point-to-point interconnections between transmitter and receiver are supported. Thus, for communication between a digital mixer and a number of multi-channel devices, such as multi-track tape recorders, analogue-to-digital converters (ADCs), and digital-to-analogue converters (DACs) for example, separate MADI links have to be provided between each device and the mixer.
Existing multi-channel devices tend to have substantially less than 56 channels. For example, a typical ADC/DAC rack might handle 16 data channels, and multi-track tape recorders typically handle 24, 32 of 48 data channels. Since MADI supports only point-to-point interconnections, the full capacity of the MADI carrier cannot always be utilised. If a device has only 24 data channels, for example, then 32-channel capability on the point-to-point MADI link is lost.
MADI is used for digital audio signal communication only. Synchronisation between devices is achieved by means of an independently distributed master synchronising signal. A further communication link must be provided for control communication between devices. Thus, the full interconnection between a device and a digital mixer is as shown in FIG. 1 of the accompanying drawings. A pair of links 1a, 1b is required for two-way digital audio signal communication in accordance with the MADI standard. Another connection 2 is required for synchronisation between the mixer and device. Control communication between the mixer and device, ie of machine control commands, device status responses, time codes etc, is achieved by means of an independent remote control link 3 using a known remote control protocol. Thus, interconnection of the device and mixer for MADI communication requires at least four connections between the device and mixer. As a further four connections are required for each additional device connected to the mixer, the overall system connection can be extremely complex. Further, while MADI links using optical fibre cables can be several kilometers in length, known remote control protocols generally specify cable lengths significantly less than this, for example 100 meters. Thus, MADI cable lengths are limited in practice by the need for an independent control link.